Never used the Angel so I can't say about that one but I do have the Tommee Tippee one albeit the version up from the Argos one. It gives me great peace of mind and I know I'd be checking every 5 minutes otherwise.
The monitor is incredibly sensitive and will even pick up the movement of you walking across the floor. You can use it under a matress about 5-6" thick. (Ours is 5.5")
The only downsides to the Argos version is that it only has 2 channels, fine if you don't live in the centre of a busy town. My friend lives in Liverpool and can't use hers as she keeps picking up the taxi cabs on it. That's the problem with analogue. The other drawback (if you see it that way) is it doesn't have the temperature on the bas unit. It's hardly a dealbreaker but I found it really handy, but there are other stand alone temperature monitors on the market if that's important to you too.
I got mine on eBay for about £70 (retails at about £130) It has the temperature on the base unit, has 5 analogue channels and extra bits and pieces (all technological terms that escape me at the moment) that cut down noise and interference.
I would buy this again in a heartbeat if it ever went wrong.
we have the angelcare one - http://www.argos.co.uk/static/Product/partNumber/3762527/Trail/searchtext%3EANGELCARE.htm
reduced there to 59.99 and it is fantastic - we chose this over the tommee tippee one as 2 friends of ours had had the tommee one and had problems with it not being able to find the connection between the monitors (same with replacement) and also having problems with it picking up their neighbours phones!